It is desirable to use fluorescent lamps as light sources for document scanners because the lamps run cooler and use less energy than incandescent lamps. Variable intensity is required to adjust the amount of illumination for different scanning conditions.
Variable intensity control of AC driven fluorescent lamps is known but it wastes large amounts of energy in the devices that control intensity. One example is shown in an article entitled "Illumination Control Circuit" by L. M. Ernst and W. E. McCollum published in the IBM Technical Disclosure Bulletin at pages 5132-5133 in the May, 1978, issue.
This disclosure teaches an AC driven variable intensity control circuit for fluorescent lamps. A diode bridge circuit directs the current in each half cycle through power transistors that act as a variable current source. The transistors control the current through the lamp and thus the operating point even if the lamp is operating in its negative resistance region. However, the transistors must absorb and radiate large amounts of energy. Thus the circuit wastes energy.
Variable intensity control of electric discharge lamps that are direct current (DC) driven is also known. One technique for varying the intensity is to drive the current through the lamp always in the same direction while modulating the amount of drive applied during a duty cyle. Two examples of such circuits are shown in U.S. Pat. No. 3,265,930 issued to W. F. Powell Jr. and U.S. Pat. No. 3,569,775 issued to C. P. Halsted et al.
In the Powell patent, the duty cycle drive is split between a voltage source and an energy storage device. In the first portion of the cycle the lamp is driven by the voltage source and energy is stored in a capacitor or inductor. In the second portion of the cycle, the lamp is driven by the energy stored in the capacitor or inductor.
In the Halsted et al patent, the duty cycle is split between a current source and a voltage source. In one portion of the cycle the lamp is driven by a current controlled by a transistor. In the other portion of the duty cycle the lamp is driven by a trickle current through a resistor; the trickle current is just sufficient to keep the lamp on.
Both the Powell and Halsted circuits have the problem of shortening the life of the electric discharge lamp because current flow is always in the same direction. A unidirectional current flow in an electric discharge lamp causes charged particle migration in the lamp. To solve the charged particle migration problem when the lamp is DC driven, the direction of current is alternated. Two examples of alternate DC current drives for an electric discharge lamp are U.S. Pat. No. 3,707,648 issued to John Rosa and U.S. Pat. No. 4,168,453 issued to F. H. Gerhard et al.
In the Rosa patent the lamp current is supplied through an inductor from a voltage source. The inductor is used to store energy and provide drive current during a portion of the drive cycle.
In the Gerhard et al patent, the lamp current is supplied by a voltage source in one direction of flow and by an inductor in the other direction of flow. Gerhard et al use a current monitor to control the switching point between current drive from the voltage source and from the inductor.
The problem with the Gerhard et al and with the Rosa circuits is that they do not current control the energy applied to the lamp. They do use inductors that provide a short term current change limitation, but inductors alone can not control the operating point of an electric discharge lamp which has a negative resistance characteristic.
The Gerhard et al circuit does have a current monitor circuit that monitors the combined current through the lamp and the inductor. However, Gerhard et al assume that current through the lamp is constant so they can monitor current build up in the coil. For some specific lamps and specific voltage bias conditions, the lamp current may be predictable and possibly constant, but for electric discharge lamps in general it will not be predictable or constant. The difficulty with the Gerhard et al circuit is that the current operating point of the lamp is not really known or monitored by their current monitor circuit. The circuit is a voltage control circuit for a negative resistance lamp, a potentially unstable and thus nonuniform source of light.
Accordingly, the problem to be solved in building a variable intensity fluorescent lamp that is energy efficient is how to provide strict current control of the lamp while minimizing energy losses in the drive circuit.